Developmental Nicotine Exposure Alters Neurotransmission and Excitability in Hypoglossal Motoneurons

Abstract

Hypoglossal motoneurons (XII MNs) control muscles of the mammalian tongue and are rhythmically active during breathing. Acetylcholine (ACh) modulates XII MN activity by promoting the release of glutamate from neurons that express nicotinic ACh receptors (nAChRs). Chronic nicotine exposure alters nAChRs on neurons throughout the brain, including brain stem respiratory neurons. Here we test the hypothesis that developmental nicotine exposure (DNE) reduces excitatory synaptic input to XII MNs. Voltage-clamp experiments in rhythmically active medullary slices showed that the frequency of excitatory postsynaptic currents (EPSCs) onto XII MNs from DNE animals is reduced by 61% (DNE = 1.7 ± 0.4 events/s; control = 4.4 ± 0.6 events/s; P < 0.002). We also examine the intrinsic excitability of XII MNs to test whether cells from DNE animals have altered membrane properties. Current-clamp experiments showed XII MNs from DNE animals had higher intrinsic excitability, as evaluated by measuring their response to injected current. DNE cells had high-input resistances (DNE = 131.9 ± 13.7 MΩ, control = 78.6 ± 9.7 MΩ, P < 0.008), began firing at lower current levels (DNE = 144 ± 22 pA, control = 351 ± 45 pA, P < 0.003), and exhibited higher frequency-current gain values (DNE = 0.087 ± 0.012 Hz/pA, control = 0.050 ± 0.004 Hz/pA, P < 0.02). Taken together, our data show previously unreported effects of DNE on XII MN function and may also help to explain the association between DNE and the incidence of central and obstructive apneas.

Increased nicotinic receptor desensitization in hypoglossal motor neurons following chronic developmental nicotine exposure

“…XII MNs were large (20 -60 m) and were localized in thick transverse brainstem slices by their anatomy, location, and physiology, as previously reported (Fig. 1, A and B) (Funk et al 1993;Pilarski et al 2010;Tan et al 2010). In these thick slices, XII MNs receive rhythmic Fig.…”

“…Studies were done using neonatal rats that were exposed to either saline or nicotine throughout gestation, and for the first week of life (via breast milk), as described previously (Fregosi and Pilarski 2008;Pilarski and Fregosi 2009;Pilarski et al 2010). We studied neonates of either sex that ranged in developmental age from 1 to 4 postnatal (P) days (i.e., P1-P4) with a body weight range of 7-12 g. Wherever possible we minimized the number of animals necessary to assess treatment effects.…”

“…Transverse medullary slices are a widely accepted experimental model for understanding breathing control (for review, see Rekling and Feldman 1998) and were produced as described previously (Pilarski et al 2010). Briefly, the brainstem was isolated and sectioned in a rostral to caudal orientation in ice-cold artificial cerebral spinal fluid (aCSF).…”

“…Current and voltage signals were amplified and acquired as previously described (Pilarski et al 2010). R in was calculated for each cell by measuring the current response evoked by rapid 10-mV depolarizing steps from a holding potential of Ϫ70 mV.…”

“…In an effort to address these issues, Robinson et al (2002), Eugenin et al (2008), Kamendi et al (2009), Pilarski and Fregosi (2009), and Pilarski et al (2010) showed that chronic developmental nicotine exposure (DNE) is associated with abnormal nicotinic cholinergic neurotransmission in brainstem respiratory neurons. Yet, it is not clear whether these results are due to agonist-induced nAChR desensitization, secondary effects of nicotine exposure, or both.…”